Air-operated seismic gas exploders

ABSTRACT

This invention relates to marine seismic gas exploders and to methods for operating them. A seismic gas exploder typically includes a housing having an expansible combustion chamber to which is periodically supplied a charge of a combustible gas mixture. The charge is detonated and the spent gases are preferably exhausted by a vacuum-operated exhaust system. The combustible gas mixture in accordance with this invention generally includes oxygen, a fuel gas, and an inert gas or preferably a fuel gas, oxygen and air. The partial air pressure is selected to allow the oxygen to completely burn the fuel gas and the nitrogen portion of the air is selected to obtain a prefiring pressure equal to or less than the ambient pressure of the water.

United States Patent [72] lnventors BenB.Thigpm; 3,480,l0l 11/1969 Barry etal. 181/5 1c M A l N gsh ofnoustonTex' Primary Examiner-Rodney D. Bennett, Jr. Ma 16 1969 Assistant Examiner-H. A. Birmiel gf Attorneys-Michael P. Breston, Alan C.Rose and Alfred B.

[73] Assignee Western Geophydcal Company of America Le-ylile- Houstom'l'u.

[54] All-OPERATED SEISMIC GAS EXPLODERS ploders and to methods for operating them. A seismic gas exmfl" ploder typically includes a housing having an expansible com- 7 C 3 Driving bustion chamber to which is periodically supplied a charge of a combustible gas mixture The charge is detonated and the fl 11/00 spent gases are preferably exhausted by a vacuum-operated 1C exhaust ystem The combustibe gas mixture in accordance I NC with this invention generally includes oxygen, a fuel gas, and

' an inert gas or preferably a fuel gas, oxygen and air. The par- [56] (chm CM tial air pressure is selected to allow the oxygen to completely UNITED STATES PATENTS burn the fuel gas and the nitrogen portion of the air is selected 3,055,450 9/1962 Richards 181/.5 lC to obtain a prefiring pressure equal to or less than the ambient 3,l76,787 4/1965 Roever 181/.5 IC pressure of the water.

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l I l 0 To T. T: T5 T4 T5 m FILL 238 FIRE EXHAUST 3 TI M E ATTORNEY The use of gas exploders as seismic energy sourceshas in recent years become widespread. One such gas exploder, known in the art-as the AQUAPULSE source (trademark of assignee), includes an 'elastic, i nflatable cylindrical sleeve or boot coupled to the outlet of a combustion chamber assembly. Aplurality of relatively small-diameter metallic tubes are arranged parallel to and at an equal distancefrom the longitudinal axis of the sleeve to fonn a cylindrical cage. The tubes allow water to flow therethrough during movement of the exploder. Prior to the detonation the elastic sleeve is flattened against and around the cage. The confined volume inside the collapsed sleeve is the gas-mixture-containing volume and formsa confmed combustion chamber. An AQUAPULSE source of the foregoing type is described in U.S. Pat. No. 3,480,101. 1

An improved AQUAPULSE source includes a vacuumoperated exhaust system for venting the spent gases from the combustion chamber subsequent to each detonation. Such an exhaust system is described in cope nding patent application Ser. No. 779,931, assigned to the same assignee.

To allow the use of gas exploders of relatively large dimenchamber a center core either solid or hollow. A combustion chamber with such a core isdescribed in copending patent application Ser. No. 825 ,380, also assigned to the same assignee; Known AQUAPULSE seismic gas exploders are operated with oxygen as the oxidizer gas and with propane as'the fuel gas. In exploders having relatively large volume combustion chambers whichare exhausted by a vacuum-operated exhaust line, the pressure in the combustion chamber after the filling cycle is relatively low as compared to the desired optimum prefiring pressure.

Marine seismic crews utilizing gas exploders as seismic energy sources ordinarily remain at sea for as long as their fuel and oxygen supplies last The supply of oxygen is usually used up first. Since while the boat is being restocked no seismic explorations are carried out, it will be appreciated that this invention significantlyreduces the requirements for pure oxygen and achieves great economies.

To obtain a fastrising acoustic impulse from a gas exploder especially of the AQUAPULSE type, it is desired that the combustible gasv mixture pressure immediately prior to detonation have an optimum value. If the predetonation pres sure is greater than the ambient pressure of the water surrounding the sleeve, the sleeve will stretch and a portion of the energy produced by the detonation will be used up in overcoming the resistance ofiered by the stretched sleeve. On the other hand, if the predetonation pressure is too low inefficient detonation 'and hence inefiicient energy transfer takes place.

FIG. I is a block diagram representation of a seismic, gas exploder system embodying the present invention;

FIG. 2 is'a cross-sectional view of a preferred gas exploder apparatus which can be used with the system shown in FIG. 1;

and

FIG. 3 is a pressure-time diagram of the pressure in the combustion chamber in the apparatus shown in FIG. 2.

Referring now to FIG. 1, there is shown a seismic gas exploder system, generally designated by the reference character 10. The system typically includes a confined variable volume combustion chamber 12 represented schematisions, it isadvantageous to provide in the combustion cally bydotted lines inside a gas exploder 11. A pressurized gas mixture is introduced into combustion chamber 12. A firing system, which may include one or more spark plugs 14 mounted in combustion chamber 12, is used to detonate the gaseous mixture. Spark plug 14 is supplied through a wire 16 with electrical energy from a power and timing control unit 18. An intake valve 20 is actuated by a solenoid 22 which is energized through a wire 24 by the control unit 18. Valve 20 controls the intake of fuelgas such as propane or other volatile hydrocarbon from a fuel supply source 26 feeding a line 28. An intake valve 30 is operated'by a solenoid 32 which is energized through a wire 34 connected to the power and timing control unit 18. Valve 30 controlsthe intake of an ox,- idizer gas such-as oxygen from a line 36 coupled to an oxygen source 38. Another intake valve 40 is operated by a solenoid 42 which is energized through a wire 44 connected to the power and timing control unit 18. Valve 40 controls the intake of compressed air from a line 46 coupled to a compressed air reservoir 48. A

,Combustion chamber 12 may be provided with a mechanical exhaust valve but preferably is provided with a valve 50 which is controlled by a solenoid-operated pilot valve 53 actuated by a solenoid 52. Solenoid 52 is controlled by a wire 54 connected to the power and timing control unit 18. The exhaust system is described in copending application Ser. No. 814,022. Valve 50 sends the exhaust-gases through a line 56 into a vacuum chamber orreservoir 60. Valves 20, 30, 40 and 50 are suitably mounted on an end wall 62 forming part of or coupling to exploder l l.

Also, the wave shape of the resulting seismic impulses is rela- SUMMARY OF THE INVENTION In accordance with the present invention, a mixture of fuel, oxygen, and air is formed. The partial pressure of the fuel gas is selected to obtain the desired useful output acoustic energy. The partial pressure of the total oxygen (pure oxygen plus oxygen in air) is selected to completely oxidize the fuel gas. The partial pressure of air is selected so that the sum of the partial pressures of the nitrogen portion of the residual spent gases, from the previous detonation, oxygen, air, and fuel is equal to or less than the ambient pressure of the water surrounding the gas exploder. i

In a preferred gas exploder system, supply sources of air, oxygen, and fuel are provided to an AQUAPULSE gas exploder having an elastic boot acting as an acoustic radiating surface.

The various gas lines 28, 36, 46, and 56 as well as the electric wires 16, 24, 34, 44, and 54 are suitably grouped into a cylindrical resilient pipe 64. The vacuum reservoir. 60 is ship (not shown).

For a better understanding of this invention reference is made to FIG. 2 wherein is shown a preferred embodiment of a marine energy source or gas exploder, generally designated by reference character 110. Exploder includes an elastic, inflatable sleeve or boot 112 made of a suitable rubber material. The sleeve's free ends 114 and 1 16 are respectively secured to U-shaped end members 118 and 120 made of metal. Members 118 and 120 have flat wall portions 122, 124 disposed substantially perpendicularly to the longitudinal axis 125 of exploder 110. Sleeve 112 is fastened to end members 118 and 120 by suitable clamps 126.

To prevent sleeve 112 from flattening after each detonation and to prevent heat damage, there is provided a cylindrical grid or cage 128 which includes a plurality of metallic tubes 130. Tubes 130 are disposed in spaced-apart relationship equally distant from and parallel to the longitudinal axis 125. Tubes 130 are of relatively small diameter and the inter-tubular spacings are not wide enough to allow sleeve 112 to penetrate between the tubes when sleeve 112 is in its collapsed condition against cage 128. This condition is caused by the surrounding hydrostatic pressure subsequent to the decrease of the high-pressure produced by the internal gas combustion. Being made of metal, tubes 130 are thermally conductive. The flow of water through tubes 130 when exploder 110 is moved, cools the metal and prevents heat damage to the rubber boot 1 12.

A center, hollow core, generally designated as 135, is provided which extends between and is supported by the end walls 122 and 124. To maintain the symmetry of construction, core 135 is cylindrical throughout its length except for its end portions which may be funneled to provide additional space on the end walls 122 and 124 useful for mounting accessory parts. To structurally support tubes 130 in the form of the cylindrical grid or cage 128, a plurality of spaced-apart annular serrated spacers 146 are disposed along the longitudinal axis 125. The end walls 122 and 124 support tubes 130 and spacers 146. Since sleeve 112 in its collapsed condition is adjacent to and totally surrounds cage 128, as shown by the dotted lines 164, the volume inside boot 112 now available for the combustible mixture of gases to enter is the volume between the outer surface of core 135 and the inner surface of the wall of collapsed sleeve 112. This volume is referred to as the combustion chamber 140.

Coupled to end wall 122 is a suitable mixing chamber 150 which receives fuel, oxygen and air supplied by lines 28, 36 and 46, respectively. Mixing chamber 150 allows the oxygen, air and propane to become thoroughly mixed. From mixing chamber 150 an outlet port 160 extending through an opening in end wall 122 allows the thoroughly mixed combustible gas mixture to enter and fill combustion chamber 140. For each seismic shot" the partial pressures of the admitted gases and air are predetermined. The products of combustion are vented from the combustion chamber 140-through vent line 56 to vacuum reservoir 60 on board ship.

A suitable ignition device such as a spark plug 162 is conveniently mounted in a wall of mixing chamber 150. In opera- -tion, an ignition pulse is transmitted from the deck of the seismic boat (not shown) through a cable 163. After the end of the filling period, the combustible gas mixture is detonated by spark plug 162. The detonation produces an expansion of the gases in combustion chamber 140 which expansion is accompanied by a steep pressure increase. The rubber boot 112 becomes inflated and stretched outwardly against the surrounding hydrostatic pressure. The stretched position of sleever 112 is shown by the dotted lines 166. The nearly instantaneous expansion of sleeve 112 transmits a sharp acoustic pulse into the surrounding water medium. Thus, a portion of the energy generated by the combustion of the fuel is transformed by exploder l into useful acoustic energy.

With particular reference to FIG. 3, just prior to the opening of inlet valves 20, 30 and 40, that is, just prior to T the pressure inside the combustion chamber 140 is at its lowest level and is below atmospheric pressure. At T the exhaust valve 50 is closed and the inlet valves are opened to introduce a fresh charge of a combustible gas mixture and to allow the pressure inside chamber 140 to graduallyincrease until, at a time T,, it reaches its predetermined prefiring pressure level which in accordance with the'present invention is equal to or less than the ambient water pressure at the operating depth of exploder 110. Since the time of opening of exhaust valve 50 is remotely controlled, the prefiring pressure level can be varied by varying the supply pressure in each of the gas sources 26, 38 and 48.

At a time T an ignition pulse is applied to plug 162 which ignites the combustible charge in combustion chamber 140. The combustion spreads throughout the entire combustion chamber very rapidly. From time T to a time T;, (on the order of a few milliseconds) there is a sharp pressure rise inside the combustion chamber 140 which rapidly decreases because of cooling water circulating through pipes 130. At a time T the exhaust valve 50 is caused to open by energizing (or deenergizing) solenoid 52. A purging of the products of combustion assists in the purging process. At a time T the exhaust valve 50 is caused to close.

It will be appreciated that lines 28, 36 and 46 supply fuel and oxidizer gases, at the beginning of each cycle, into an evacuated combustion chamber 140. Without the nitrogen partial pressure supplied by air source 48, the prefiring pressure would not be adequate for efficient combustion. The addition of air in the proper amount improves the overall efficiency of the seismic system. This can be illustrated with the following examples.

Example No. 1: If an AQUAPULSE source is operated at a depth of 40 feet below the surface of the sea, the absolute ambient pressure of the sea at this depth is on the order of 33 p.s.i. In the vacuum-exhausted AQUAPULSE source, the pressure of the spent gases inside the combustion chamber before the fueling cycle begins (prior to T is substantially less than 1 p.s.i.. Compressed air is inserted to a partial pressure of approximately 25 p.s.i., propane to a partial pressure of approximately 1.5 p.s.i., and oxygen to a partial pressure of 4 p.s.i. The total pressure inside the combustion chamber will then be on the order of 31.5 p.s.i., approximately 1.5 p.s.i. less than the ambient hydrostatic pressure at a 40-foot depth. The partial pressure of oxygen resulting from the combination of compressed air and pure oxygen will be on the order of 9 p.s.i. (4 p.s.i. of pure oxygen and one-fifth of 25 p.s.i. of air). The 9 p.s.i. of oxygen will be more than adequate for the complete combustion of the 1.5 p.s.i. propane. The approximately 20 p.s.i. (four-fifths of 25) of nitrogen, which is an inert gas, does not interfere with the combustion of the propane but raises the preignition pressure of the combustible mixture without stretching sleeve 112.

Example No. 2: If it is now assumed that the AQUAPULSE source is operated at a depth of 60 feet below the surface of the sea the absolute ambient pressure of the sea at this depth is on the order of 42 p.s.i. Compressed air is inserted to a partial pressure of approximately 37 p.s.i., propane to a partial pressure of 1.5 p.s.i., and oxygen to a partial pressure of 1.5 p.s.i. The pressure of the spent gases in the vacuum-operated exhaust system is still less than 1 p.s.i. The total pressure inside the source will then be on the order of 41 p.s.i., approximately 1 p.s.i. less than the ambient hydrostatic pressure.

From the above examples it will be apparent that the greater the ambient pressure the less pure oxygen is needed. In relatively deep waters (say 70 feet) the need for pure oxygen can be completely eliminated.

Since the preignition pressure is now nearly equal to ambient upon detonation, the pressure inside the combustion chamber almost instantaneously exceeds the external ambient pressure. In a conventional gas exploder, not employing compressed air as described herein, an appreciable pressure buildup time is required, especially when the total pressure of the combined gas mixture in the combustion chamber is less than half of the ambient pressure.

As a consequence of building up the internal pressure to a value substantially equal to the ambient pressure, there is obtained a more immediate rise time which will have the beneficial effect of improving the acoustic spectrum produced by the gas exploder. A greater internal pressure will also improve the exploder's efficiency, by improving the impedance match across the source boundary.

Since the depth of the exploder can normally not be maintained to an accuracy better than plus or minus 2 feet, it is preferable for the sum of the partial pressures to be always slightly less than ambient pressure. If it is greater than ambient sleeve 112 will stretch prior to firing and a portion of the energy will be used up in overcoming the resistance of the sleeve.

While this invention has been described in terms of a mixture of propane, oxygen, and air; it is apparent that mixtures of other gases can be used to produce identical final compositions and pressure for detonation. For example, the combustion chamber may be supplied with propane, oxygen, and nitrogen to produce the desired total pressure and partial pressures of propane and oxygen.

Similarly, if oxygen-enriched air (or impure oxygenlcan be obtained, it may be used to supply the necessary partial pressure of oxygen. In the case where the ambient operating pressure is 40 feet, a suitable final mixture could be obtained with 1.5 p.s.i. of propane'and 29 p.s.i. of enriched air having onethird oxygen which with l p.s.i. of spent gas, produces a total pressure of 3 1.5 p.s.i. in which the partial pressure of oxygen is approximately 9.7 p.s.i. and that of the inert gases is 29.3 p.s.i.

,What we claim is:

1. A seismic gas exploder system for generating pulses of energy in a body of water when the exploder moves through the water at depths having predetermined ambient pressures, the exploder comprising:

a housing having an extensible wall enclosing a confined expansible combustion chamber; supply means to supply a combustible gaseous mixture to said chamber, said mixture including compressed air, oxygen and fuel, the partial pressure at least of the nitrogen portion of said air being determined in dependence upon the ambient pressure of f the water at the depth at which said exploder operates; ignition means to detonate said mixture within said chamber thereby producing spent gaseous products of combustion which causes said chamber to expand and to generate said pulses; and exhaust means operatively coupled to said chamber and arranged to purge said gaseous products of combustion from said chamber.

2. The gas exploder system of claim 1 wherein the sum of the partial pressures of said fuel, air and oxygen and of the partial pressure of said spent products remaining in said chamber prior to detonation is substantially equal to said ambient pressure.

i 3. The gas exploder system of claim 1 wherein the sum of said partial pressures and of the partial pressure of said spend pendence upon said ambient pressure.

4. The seismic gas exploder system of claim 1 wherein said exhaust means include a conduit coupling said chamber to a vacuum reservoir.

5. The gas exploder system of claim 1 wherein said extensible wall includes an elastic cylindrical sleeve,

a center core in said sleeve, and

a plurality of hollow tubes arranged substantially parallel to and at an equal distance from the longitudinal axis of said sleeve to form a cylindrical cage for supporting said sleeve subsequent to the detonation of said combustible mixture.

6. A method of energizing a gas exploder having a confined combustion chamber for generating pulses of energy in a body of water when the exploder moves through the water at depths having predetermined ambient pressures, the method comprising the steps of:

supplying a combustible gaseous mixture to said combustion chamber, said mixture including inert gas, oxygen and fuel;

adjusting the respective partial pressures of said inert gas,

- oxygen, and fuel in dependence upon the ambient pressure of the water at the depth at which said exploder operates;

detonating said combustible mixture in said chamber to produce spent gaseous products of combustion which expand said chamber and generate said pulses; and

exhausting said gaseous products of combustion from said chamber to a final spent gas pressure.

7. The method of claim 6 wherein the sum of said partial pressures of inert gas, oxygen, and fuel and of the said final spent gas pressure in said chamber is substantially equal to said ambient pressure. 

1. A seismic gas exploder system for generating pulses of energy in a body of water when the exploder moves through the water at depths having predetermined ambient pressures, the exploder comprising: a housing having an extensible wall enclosing a confined expansible combustion chamber; supply means to supply a combustible gaseous mixture to said chamber, said mixture including compressed air, oxygen and fuel, the partial pressure at least of the nitrogen portion of said air being determined in dependence upon the ambient pressure of the water at the depth at which said exploder operates; ignition means to detonate said mixture within said chamber thereby producing spent gaseous products of combustion which causes said chamber to expand and to generate said pulses; and exhaust means operatively coupled to said chamber and arranged to purge said gaseous products of combustion from said chamber.
 2. The gas exploder system of claim 1 wherein the sum of the partial pressures of said fuel, air and oxygen and of the partial pressure of said spent products remaining in said chamber prior to detonation is substantially equal to said ambient pressure.
 3. The gas exploder system of claim 1 wherein the sum of said partial pressures and of the partial pressure of said spend products remaining in said chamber prior to detonation is less than said ambient pressure by a predetermined amount in dependence upon said ambient pressure.
 4. The seismic gas exploder system of claim 1 wherein said exhaust meaNs include a conduit coupling said chamber to a vacuum reservoir.
 5. The gas exploder system of claim 1 wherein said extensible wall includes an elastic cylindrical sleeve, a center core in said sleeve, and a plurality of hollow tubes arranged substantially parallel to and at an equal distance from the longitudinal axis of said sleeve to form a cylindrical cage for supporting said sleeve subsequent to the detonation of said combustible mixture.
 6. A method of energizing a gas exploder having a confined combustion chamber for generating pulses of energy in a body of water when the exploder moves through the water at depths having predetermined ambient pressures, the method comprising the steps of: supplying a combustible gaseous mixture to said combustion chamber, said mixture including inert gas, oxygen and fuel; adjusting the respective partial pressures of said inert gas, oxygen, and fuel in dependence upon the ambient pressure of the water at the depth at which said exploder operates; detonating said combustible mixture in said chamber to produce spent gaseous products of combustion which expand said chamber and generate said pulses; and exhausting said gaseous products of combustion from said chamber to a final spent gas pressure.
 7. The method of claim 6 wherein the sum of said partial pressures of inert gas, oxygen, and fuel and of the said final spent gas pressure in said chamber is substantially equal to said ambient pressure. 